ISP throttling on the PPP interface.

dE_logics · Advocate Joined: 02 Jan 2009 Posts: 2289 Location: $TERM

I was wondering how the PPP interface works.

Suppose, the ISP provides the Internet connection via PPP; Linux spawns a PPP interface to get access to the connection and the speed of the connection is limited not to the medium (like Ethernet, wireless etc...), but by the ISP at it's point of presence.

In case the rate of incoming packets (from the Internet) or outgoing packets is higher than the speed the ISP limits, where will the input/output packet queue be placed? On the PPP interface or on the ISP's end?
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NeddySeagoon · Posted: Thu May 15, 2014 2:20 pm Post subject:

dE_logics,

There won't be a queue. If your ISP, or a carrier does throttling, its by dropping your outgoing packets so they do not receive responses any faster than they can//want to deliver them to you.
This avoids them discarding data that has already travelled (uselessly) over their network

Its interesting that you mention PPP. The MTU for PPPoE is 1492, as the PPP header (eight bytes) has to fit into a standard Ethernet packet.
This means that all your interfaces that originate packets that may travel over a PPPoE link should use a MTU of 1492, not the default 1500.
This avoids packet fragmentation and reassembly which will have a significant impact on your data rate when the system is trying to send packets >1492 bytes.
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szatox · Advocate Joined: 27 Aug 2013 Posts: 3491

if it's ISP that limits speed (and usualy it is), he limits it on his end. He can't even access your end, can he? At least he shouldn't.
Anyway, check out HTB for details (hieracial token bucket) - ofcourse it's not the only way, but the idea behind it will be pretty much the same regardless of particualr implementation.

Also, some ISPs only care for how fast you can download and don't even bother with throtling your upload speed, so they only need the easy part with outgoing packets (From their point of view - those you will se as incoming). Limiting speed of incoming packets seems to be a bit more tricky and might require looping them over virtual network interface so you can limit them in outgoing queue as well.

dE_logics · Advocate Joined: 02 Jan 2009 Posts: 2289 Location: $TERM

dE_logics · Advocate Joined: 02 Jan 2009 Posts: 2289 Location: $TERM

NeddySeagoon · Posted: Thu May 15, 2014 9:00 pm Post subject:

dE_logics,

Your downlink cannot be throttled - there is nothing special about PPP there.
There is no queue either.

To avoid the need for queueing, the uplink is controlled by dropping packets so that data does not arrive at network boundary faster than it can be delivered to you.
This makes your network resend the packets that get dropped.

Your router will be splitting packets bigger that 1492 bytes into two that you send and reassembling any fragments that it receives from the outside world.
However, there is a process called link MTU discovery, which if it works will set the MTU actually used to the minimum MTU supported along the route, so no fragmentation occurs anywhere.
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NeddySeagoon

Computer users fall into two groups:-
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dE_logics · Advocate Joined: 02 Jan 2009 Posts: 2289 Location: $TERM

krinn · Watchman Joined: 02 May 2003 Posts: 7470

NeddySeagoon · Posted: Fri May 16, 2014 1:16 pm Post subject:

dE_logics,

You can discover the link MTU with ping.
Send various size packets with the 'do not fragment' bit set. The largest packet that gets a non error response in the link MTU.

Whatever you discover is a good MTU setting.
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NeddySeagoon

Computer users fall into two groups:-
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those that have never had a hard drive fail.

krinn · Watchman Joined: 02 May 2003 Posts: 7470

There's no real need to make any MTU discovery for PPP, the size standard is set to 1492.

dE_logics · Advocate Joined: 02 Jan 2009 Posts: 2289 Location: $TERM

It's 1440 here.
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NeddySeagoon · Posted: Sat May 17, 2014 11:06 am Post subject:

dE_logics,

1440 means that somewhere along the route you tested is a node with a MTU of 1440.
Thats a good value to set for all of the interfaces on your network.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

dE_logics · Advocate Joined: 02 Jan 2009 Posts: 2289 Location: $TERM

Thanks for clarifying all of this.
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krinn · Watchman Joined: 02 May 2003 Posts: 7470

Well, you better ask your ISP about it.
If it disable MTU discovery and use non standard size, there's something wrong then.

NeddySeagoon · Posted: Sun May 18, 2014 12:27 pm Post subject:

krinn,

That's a whole new nest of vipers.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.